Although the first useful lasers were developed in the 1960s, recent advances in laser and fiber optic delivery systems have greatly enhanced the use of this technology in the field of medicine. Today there are numerous types of laser systems designed for operation in a wide range of applications primarily related to surgical and other medical procedures.
Laser fibers are used in different ways, including incision, necrosis or killing of live tissue, excision or removal of tissue and structure, and cauterization of tissue. A very focused beam would provide the greatest amount of control during either operation. Cauterization and necrosis of living tissue is accomplished by coagulation, or more precisely with respect to the laser itself, by photocoagulation of contacted or penetrated tissue. In this process the laser beam causes the proteins in the contacted tissue to heat up rapidly and thermally denature. This essentially kills living tissue and seals blood vessels. The process has been likened to frying an egg. In practice, during an incision procedure cauterization of the incised tissue is likely to occur simultaneously. Thus, laser surgery is often characterized by an absence of bleeding during the surgery.
A common type of laser known as a CO2 laser delivers radiation with a wavelength of 10.64 microns. However, in order to focus or channel the radiated energy produced by a CO2 laser it is necessary to configure sets of mirrors in certain ways. These systems are typically large and expensive. With the mid-1980s FDA approval of the Nd:YAG type laser delivering electromagnetic energy at a wavelength of 1.064 microns, it became possible to generate and focus the laser radiation through a silica core optical fiber. Recently, the Holmium: Yttrium-silver-garnet (Ho:YAG) laser has become an important tool in the hospital and clinic due to it's relatively low cost and wide range of applications suitable for it's use.
Lasers, and in particular Ho:YAG lasers, have been used for a variety of purposes in urology. Such procedures include performing partial or full nephrectomies (removal of one or both of the kidneys), laser-assisted trans-urethral resections of the prostate (a TURP is a procedure required for managing benign prostatic hyperplasia or BPH--a frequent condition caused by swelling of the prostate), treatment of superficial bladder carcinomas or tumors, and other laparascopic procedures.
Numerous modalities exist for the destruction of urinary tract calculi, aside from lasers. These include electrohydraulic probes, ultrasonic probes, electromechanical impactors, and the lithoclast (a compressed air-driven metal pin). Focused shock waves can be delivered from an external source in a procedure known as extracorporeal shock wave lithotripsy or ESWL.
Destruction of gall stones and urinary stones with lasers has been studied as early as 1979. The following is a brief survey of various types of lasers useful for such procedures:
Excimer lasers (xenon, fluoride and xenon chloride) produce shorter wavelength laser radiation. Standard excimer lasers have a pulse duration of 10 nanoseconds which is too short for easy transmission through fibers. Longer pulsed excimer lasers (up to 100 nanoseconds) can be transmitted through optical fibers more easily. Fragmentation of calculi is efficient but one drawback is that both tissue and stone absorb excimer wavelengths strongly, without any beneficial selectivity. The laser is also useful for tissue ablation.
Pulsed dye lasers operate efficiently at a pulse duration of 1 microsecond, which is ideal for stone fragmentation. These lasers emit at wavelengths from the ultraviolet to the red, according to the dye chosen. The maximum differential absorption between stone and ureter tissue occurs at 504 nanometer and this wavelength is, therefore, useful.
Alexandrite lasers are solid-state lasers which emit at 720 nanometer. The pulse duration is typically 10 to 100 nanoseconds, the longer durations more efficiently transmitted. The disadvantage of using the lower durations, with attending high peak pulse power, is that the fiber becomes eroded rapidly during fragmentation of a stone. The longer pulsed alexandrite lasers transmit better but fragmentation of certain calculi is still not highly efficient due to the unfavorable absorption at these wavelengths.
Titanium sapphire lasers are semiconductor lasers, and therefore have the potential to be relatively smaller and more economical than others. They emit at approximately 850 nanometer. Between 3 and 13 microseconds, shorter pulse durations are favored requiring pulse energies of 100 to 200 mJoules to fragment calculi. The action on pale calculi is relatively inefficient and the laser has a variable effect on calculi as the stones shrink and change size and composition during their destruction.
Continuous wave Nd:YAG lasers, though widely used in numerous medical and other applications, should never be used to fragment urinary calculi pulsing the laser will allow fragmentation however. The 100 microsecond laser has been described as having the capability of fragmenting uric acid and gallstones, but not pale calculi. The 10 microsecond Nd:YAG laser has been used at pulse energies of 30 mJoules via 400 micron core fibers to fragment calculi. The high peak power allows fragmentation in spite of poor absorption. At these low-pulse energies, the fiber transmission is feasible. However, the distal end of the fiber is at risk of destruction should it touch the stone. Therefore, solutions include shaping a lens at the distal end of the fiber and placing a metal cap on the end of the fiber which forms a type of shock wave.
The Ho:YAG laser produces light at a wavelength of 2,000 to 2,100 nanometer (2.0 to 2.1 microns), depending upon the precise formulation of the holmium rod, in a pulsed fashion. These wavelengths are well absorbed by water, and thus their use for stone fragmentation is not obvious. Of the devices available, the energy of each pulse and the frequency can be varied. The energy levels typically used are between 0.2 and 2.8 Joules per pulse and the frequency is typically varied between 5 and 20 Hertz. Typical pulse durations are 350 microseconds. The energy produced can be transmitted along suitable silica-based optical fibers, typically ranging in core size between 250 and 750 microns. One drawback of the Ho:YAG laser is that the regimen described has an equally powerful effect on tissue, such as the ureter, and damage can be expected should the laser fiber touch it during delivery.
Endoscopes of various designs are widely used for ureteroscopy. Rigid instruments are generally employed for the distal ureter and may be advanced into the mid- and proxima-ureters in some patients. Actively deflectable, flexible ureteroscopes can be used in the proximal portions of the ureter and the intrarenal collecting system to access most areas. A working channel of at least 2.2 French is recommended for placement of the smaller fibers, such as the 200 micron diameter fiber..
The construction of optical fibers used in surgical procedures is fairly simple. A quartz, plastic or silicone cladding is used to constrain the laser light to the quartz core. Theoretically, only a few of the entering photons are directed straight down the axis of the fiber. Transmission of the radiant beam is possible since the rest of the photons are constrained to the core of the fiber due to internal reflectance by the quartz cladding interface. Very few photons escape the fiber. The technology related to the use of silica core fibers in medical lasers is well known, e.g. B. P. McCann, Photonics Spectra, May 1990, pp 127-136.